A LNB is a component attached to the edge of a BS antenna or a CS antenna. The LNB receives a feeble signal of about 12 GHz from a satellite, and amplifies the signal with a low-noise amplifier. Then the LNB converts the signal to an intermediate frequency signal by mixing it with a local oscillation signal in a mixing circuit, and outputs to an external tuner.
FIG. 5 is a block diagram showing an electrical arrangement of LNB1 adopting a typical conventional technology. This LNB1 is destined for Europe. In the LNB1, a signal of horizontally-polarized wave is inputted to a terminal t1 and a signal of vertically-polarized wave is inputted to a terminal t2, from a receiving horn (not shown).
As it will be described later, the inputted signals are respectively amplified by LNAs (Low Noise Amp) a1 and a2 which are selectively activated by a control circuit ct1, then are inputted to a mixer m via a common high frequency filter f. Further, a local oscillation signal is inputted to the mixer m from local oscillators 1o1 and 1o2 which are selectively activated by the control circuit ct1.
The mixer m downconverts a low-band signal inputted from the terminal t1 or the terminal t2 to an intermediate frequency signal of 950 MHz to 1950 MHz, by mixing it with a local oscillation signal of 9.75 GHz, which is inputted from the local oscillator 1o1. Meanwhile, the mixer m downconverts a high-band signal inputted from the terminal t1 or the terminal t2 to an intermediate frequency signal of 1150 MHz to 2150 MHz, by mixing it with a local oscillation signal of 10.6 GHz, which is inputted from the local oscillator 1o2.
The intermediate frequency signal is outputted to a signal input-output terminal t3 via two stages IF (intermediate frequency) amplifiers a11 and a12 and a coupling capacitor c1. Further, the intermediate frequency signal is inputted to an external tuner tu from the signal input-output terminal t3 via a co-axial cable k.
Meanwhile, a direct-current voltage, used as a power source and for switching polarization wave, and a control signal for selecting a receiving band are inputted to the signal input-output terminal t3 from the tuner tu via the co-axial cable k. The control signal is a pulse signal having the maximum amplitude Vs of 0.8 Vp-p and the frequency f of 22 kHz, for example. The control signal is inputted to the control circuit ct1 via a low-pass filter which cuts off the intermediate frequency signal. The low-pass filter is made up of an inductor 1 and a capacitor c2.
The control circuit ct1 selectively activates the local oscillators 1o1 and 1o2 in response to the control signal. The direct-current voltage is inputted to a regulator ICrg and the control circuit ct1 via the low-pass filter. The regulator ICrg stabilizes the supplied direct-current voltage to be a predetermined voltage and supplies the voltage to respective circuits as the power source. The regulator LCrg is equipped with an input capacitor and an output capacitor (both not shown) so as to compose a power supply circuit. The control circuit ct1 switches between the LNAs a1 and a2 according to the inputted direct-current voltage, which are either of 13V or 18V.
In the LNB1 having the foregoing arrangement, conventionally, a single tuner is connected to a single LNB when receiving services (such as an image). However, as the development of services, such as provision of larger number of channels, various arrangements of the system has been realized. For example, a single LNB receives plural satellites, or a single LNB is connected to plural tuners.
Therefore, a new type of LNB has become popular, which is arranged such that the pulse signal inputted from the tuner tu is used for making selection of satellites when plural satellites are received with a single LNB, or for making a selection of plural polarization waves received with the LNB, and a detector circuit provided in the LNB1 (included in the described control circuit ct1) performs switching operation for making those selections (For example, see Japanese Patent Publication No. 2988844 (published on Nov. 5, 1996)).
In this type of LNB, the amplitude WL of the pulse signal at the signal input-output circuit t3 is denoted by the following equation,WL=Wt×XL÷(XL+Xc+Xt)where:
XL represents impedance at the signal input-output circuit t3 with respect to a frequency of the pulse signal;
Xt represents impedance in the tuner tu with respect to the frequency of the pulse signal;
Xc represents impedance in a transmission line such as the co-axial cable k provided between the LNB1 and the tuner tu; and
Wt represents the amplitude of the pulse signal outputted from the tuner tu.
Generally, the impedance XL is dozens of Ω, and the impedance Xt is several Ω.
Accordingly, when a switching device 3 is provided between the plural LNBs1 and the tuner tu as shown in FIG. 6 so as to carry out switching operation for the intermediate frequency signal from the LNBs1, or, when a dividing device 4 is provided as shown in FIG. 7 so as to divide the intermediate frequency signal from a single LNB1 to plural tuners tu, in order to construct a satellite receiving system, the impedance Xc in a transmission line may increase. In this case, the amplitude WL of the pulse signal at the signal input-output circuit t3 becomes lower than the reference amplitude value of the pulse detector circuit. It may result in error operation so that the desired image signal cannot be selected.
Further, the satellite receiving system having the arrangement shown in FIG. 5 in which a single LNB1 and a single tuner tu are directly connected, and the switching device 3 or the dividing device 4 are not provided, the error detection is still possible to occur under inadequate circumstances such as the case where the co-axis cable k is long, where the pulse signal draws blunt waveforms, or where the impedance Xt in the tuner tu is high because of poor condition of the device. Accordingly, when the switching device 3 or the dividing device 4 are provided, the occurrence of error detection becomes more likely, thereby causing customer complaints even when there is no problems on the LNB1 itself.